Printing plate supports, particularly, lithographic printing plate supports are made of aluminum or aluminum alloys.
In general, aluminum plates used as lithographic printing plate supports are required to have moderate adhesion to photosensitive materials and moderate water receptivity, and further, uniformly roughened surfaces. The uniformly roughened surfaces mean that the surfaces possess pits which are moderately uniform in size and such pits are uniformly formed throughout the surfaces. The pits have a marked effect on printability of plate materials such as scumming reduction and printing durability, and the conditions of the pits are an important element in the manufacture of the plate materials.
A process for manufacturing lithographic printing plate supports which comprises graining a plate material with a brush having large stiffness to form a rough surface with directional properties followed by roughening mechanically the surface is proposned in JP-A-7-137474 (The term "JP-A" as used herein means an "unexamined published Japanese patent application").
Although the proposal made by JP-A-7-137474 excellently prevents generating defective products owing to defects such as scratching which aluminum itself has, the proposal however requires an additional step for forming a rough surface with the directional properties to incur very high cost, and in addition, the plate material thus obtained has the disadvantage of having poor scumming reduction especially on squeezing water out, because of the directional properties.